Post-Main Sequence Evolution of Our Sun
Currently, the composition of our sun's core is believed to be 35% hydrogen, 63% helium and 2% metals. This is in contrast to the Sun's zero-age composition (and the composition of the radiative and convective envelopes) of 73% hydrogen, 25% helium and 2% metals. This ratio of abundances compares favorably with that of other Population I stars.
Evolution to the Giant Branch
The Sun will leave the main sequence when its core hydrogen abundance drops to approximately 12%. At that point a thick hydrogen burning shell will surround the inert helium core. As the thick shell continues to burn, it will narrow and the Sun's envelope will expand to about 2.5 times the current solar size and its photosphere will decrease in temperature to around 5,000K.
The triple-alpha process
After the hydrogen shell is depleted, electron degeneracy sets up in the inert helium core and drives the core temperature over 100 million Kelvin. At that extreme temperature, a new round of fusion is initiated via the triple-alpha process (see figure at right.) This reaction involves the fusion of three helium nuclei into carbon and marks the star's post-main sequence life as a red giant star. The red giant stage will last for about 1/10 of the Sun's main sequence life.
The figure below depicts the solar interior as it is believed to exist today and how it may appear after the Sun reaches the red giant branch of the H-R diagram:
Evolution to the Supergiant Branch
Upon depletion of the helium in the red giant's core, the future sun will be at another crossroad between core collapse and finding a new source of energy. Unfortunately, the Sun's low mass cannot generate a high enough core temperature to initiate a further round of fusion using the newly minted carbon and oxygen in its core. The only energy source available is the hydrogen and helium surrounding the inert carbon/oxygen core: shells of hydrogen and helium begin fusing in a last-ditch attempt to stave off core collapse. Reacting to the increased luminosity from the two fusion shells, the Sun will expand to a supergiant.
As the shells narrow, however, the helium shell becomes unstable and undergoes a serious of "shell flashes" that will cause the supergiant sun to expand and contract over 100,000 year intervals. At some time during one of those shell flashes, the convective and radiative zones will expand greater than the star's escape velocity and slough off. The expanding shells will become visible as a planetary nebula leaving the bare carbon/oxygen core exposed at the center. The exposed core is a white dwarf, and after about 25,000 years, the planetary nebula will fade to leave only the white dwarf behind. The figure below depicts the Sun's evolution—specifically, its change in radius—on a logrithmic scale of time:
